Electric ship propulsion system



Aug. 29, 1944.

F. E. CREVER 2,357,086

ELECTRIC SHIP PROPULSION SYSTEM Filed Feb. 26, 1943 Inventor Frederick E. Craven 7 His Attorney.

Patented Aug. 29, 1944 ELECTRIC SHIP PROPULSION SYSTEM Frederich E. Crever, Scotia, N. Y., assignor to General Electric Company, a corporation of New'York Applicatlon Febrnary 26, 1943, Serial No. 477,258

9 Claims.

My invention relates to the control and regu-' lation of variable speed synchronous motors for the propulsion of ships and more particularly "to such control which provides stable starting.

are required to function within a wide range of speeds and loads and to withstand sudden changes of load. The sudden changes ofload may be caused by conditions prevailing during high seas or when the ship is making a turn, or when it is being reversed. When synchronous motors are employed to drivethe propellers of a ship, it is common practice to bring the motors up to their minimum synchronous speed by induction motor action. In order to meet the requirements of the wide range of speeds and loads, it is desirable to provide a regulator which will maintain a predetermined stable condition of operation, and it is also desirable that the regulator be placed in operation automatically. For example, a regulator may be employed that controls the field excitation "oi the motor and generator system to provide predetermined values of generator voltage. If the regulator is placed in operation when the system is started, it will allow the current of the generator to increase to a value as much as twice normal. This is a desirable condition since increased excitation facilitates the starting of the motor as an induction motor. However, as the motor comes up. to speed and its terminal voltage falls, the regulater tends to reduce the excitation which begins to approach the normal value determined by the regulator setting. As a result, there is dangerthat the motor will fall out of step or drop its load. Accordingly, it is an object of my invention to provide a ship propulsion system employing a synchronous generator and a synchronous motor and including an improved arrangement for starting the motor and for regulating its synchronism at low speeds, it is necessary that the automatic regulator be able to vary the excitation in accordance with the predetermined desired characteristics. For example, the system may be operated so that the ratio of the generator voltage to its frequency remains substantially constant. However, it may be desirable to vary this characteristic during operation at lower speeds. v

It is a further object of my invention to provide a ship propulsion system employing a synchronous generator and motor and including an improved regulator for maintaining a predetermined stable characteristic throughout the range of speeds of the system.

Further'obiects and advantages of my invention will become apparent as the following description proceeds, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For better understanding oi. my invention ref erence may be had to the accompanying drawing, the single figure of which represents schematically an electric ship propulsion system embodying my invention. l

Briefiy the ship propulsion system shown 0 the drawing comprises an alternating current generator driven by a variable speed steam turbine and connected to supply power to a synchronous motor. A regulator is provided for maintaining a predetermined characteristic, for example, a constant ratio between the voltage of the generator and its frequency. The regulation and excitation system includes a main exciter and a sub-exciter. The sub-exciter is a special direct current amplifying generator which, for convenience, is referred to herein as an Amplidyne. The Amplidyne has a very high amplification ratio and a very low time constant,

and it may be controlled by simple control circuits having no moving parts and substantially no time lag. The Amplidyne is provided with a separately excited direct current field which maintains a predetermined polarity of the exrear circuit.

opulsion system illustrated ises an alternating current or a variable speed steam ngecl to supply power to a ating current motor l2 for it. Armature windings ll arranged on the .tor oi the generator it! are ccrmectec power to armature windings l2 a ranged on the stator of the motor 12;

turbine 1-5 and synchronous alt driving a propel The generator ii! is provided with a field winding i5 mounted on a rotor l1 and the motor 12 is provided with a field winding (8 mounted on a rotor ii. The rotor is is provided with a squi rel cage winding 20 to facilitate starting of the motor 12 as an induction motor. The field windings i5 and i 8 are connected in parallel through sets of slip rings 2i and 22, respectively, to be,energized'by an exciter 23. A switch 24 is provided for connecting the field winding 45 to the exciter and includes a back contact 25 for shorting this winding through a suitable resistor 28. The motor is provided with a similar switch 2i having a back contact 28 and a shorting resistor 23. The turbine 9 is provided with a throttle valve 30 for varying the speed of the turbine to change the speed of the generator and its frequency for driving the motor at corresponding speeds.

The exciter 23 is provided with a field winding 33 which is connected to be energized by a pilot exciter 34. A switch 35 is provided so that, when desired, the field 33 may be energized from a separate source of direct current, a manually operated resistance 15 being provided to control the field when it is excited from the separate source. The pilot exciter 34 is an Amplidyne and it together with its control fields is arranged to regulate the excitation of the exciter 23 in order to obtain stable operation for all loads and speeds of the system. The construction and operation of an Amplidyne generator are described in United States Letters Patent 2,227,992, granted January 7, 1941, to Ernst F. W. Alexanderson and Martin A Edwards and assigned to the same assignee as the present invention. The Amplidyne consists essentially of a conventional direct current dynamo electric machine armature and commutator 31 provided with a set of short circuited brushes 3! whose commutating axis is electrically in quadrature to the commutating axis of a set of load brushes II.

' The main operating flux of the machine, that is to say the flux which produces the generated electromotive force between the load brushes 3!, is produced by the armature reaction of the cur rent flowing'between the short circuited brushes; The normal armature reaction of the exciter is substantially neutralized by a series winding 32. The resistance of the circulating path between the short circuited brushes is relatively low and the electromotive force for producing the flow cc connected in series with 24 is provided for reversing of relatively heavy short circuit current which produces the large cross armature reaction flux is produced by magnetomotive forces resulting from the energization ofthe control windings ll, H and 42. The total magnetomotive produced by these control windings is relatively small. The control winding I is energized from a suitable source of direct current in series with a field control panel or network 43, The windings H and 42 are differentially connected boosting and bucking windings, respectively, which are energized by a regulator 44 connected through a switch 45 to a potential transformer 45 and responsive to the voltage of the generator 10.

The circuit of the regulator for energizing the boost winding H is arranged to have a linear impedance-current characteristic and comprises a rectifier energized from the potential transformer 46 through a resistance 5i; the regulator circuit for energizing the winding 42 has a non-linear impedance-current characteristic and comprises a rectifier 52 energized from the potential transformer 46 through a saturating reactor 53 which is provided with taps to determine the voltage at which it saturates. The minimum value of current in the separately excited control winding 40 is determined by a resistance 54 in the network 43. The network is also provided with resistances 55 and 55 arranged to be connected in parallel with the resistance 54 by switches 51 and 58, respectively. The control winding ll determines the polarity of the Amplidyne 34 by insuring that it will always build up its voltage in the same direction. Winding 40 is also employed as a stabilizing or anti-hunting winding, the secondaries of transformers 59 and 50 being connected in the circuit of the winding Ill so that voltages representing the rates of change of the Amplidyne and the exciter voltages, respectively, are introduced in the circuit of the winding ll. The directions of these voltages are such as to set up magnetometive forces in the field of the Amplidyne 34 which oppose the changes. The resistance of the prim'ary winding of the transformer 59 is sufficiently high that the primary winding of this transformer may be connected directly across the terminals of the Amplidyne. ance ii is provided in series with the primary winding of the transformer I in order to determine the value of the stabilizing force introduced through the transformer II.

The regulator 44 controls the pilot exciter N to maintain a. substantially constant ratio between the voltage and frequency of the generator Ill. The operation of the regulator 44 is such that equilibrium is established between the boost and buck fields H and 42 such that their resultant ampere turns are Just sufficient to excite the Amplidyne 34 to a. value which causes the generator III to produce an alternating current voltage suflicient to saturate the reactor 51. Since the voltage at which the reactor saturates varies directly with frequency the regulator may be designed to hold a constant ratio between the volt- An adjustable resistreduce the ratio oithe voltage of the generator to its frequency at lower frequencies, a series resonant circuit comprising a condenser 88, an inductance 88 and an adjustable resistance 81 may be connected across the input terminals of the" rectifier 82. This series circuit may be tuned to a frequency in the upper portion of the range so that as the frequency of the generator falls less current fiows through the resonant circuit and the current in the bucking winding 42 will be increased and therefore will reduce the voltage of the generator I8. A similar correction of the voltage-frequency characteristic may be obtained by providing a condenser 88 shunting the resistance Si in the linear circuit supplying the rectifier 88. then, as the frequency falls, the circuit becomes somewhat non-linear and the drop across the parallel circuit comprising the resistance i and condenser 88 falls and the magnetomotive force produced by the boost winding H is reduced and the voltage 01' the generator I8 is lowered accordingly. It is readily apparent that the above compensating arrangement for the linear and non-linear circuits of the regulator 44 makes it possible to obtain a wide range of voltage-frequency characteristic. Furthermore, non-linearity in the reactor 88 may be corrected by such compensating arrangements.

In order to smooth out the direct current voltage produced by the rectifier 52 and particularly to prevent high voltage peaks which might be caused by the saturation of the reactor 88, a condenser 18 is provided across theterminals oi the bucking winding 42.

A switch 88 is provided so that the regulator 44 and exciter 84 may be disconnected from the direct current source and from the potential transi'ormer 48. During synchronous operation the angle 01' displacement between the rotor of the generator l8 and the rotor o! the motor l2 varies as a function of the torque, this in turn varies the alternatingcurrent voltage of the generator. The regulator acts in response to the generator voltage to maintain the ratio of voltage to frequency substantially constant; this insures high excitation and a high margin of stability during torque peaks. Ii! the regulator were not provided,

it would be necessary to employ a continuous fixed excitation which would be relatively low with respect tothat required duringthe torque peaks. Relatively low excitation would be necessary since continuous excitation is limited by the heat dissipating capacities of the motor and generator fields: the system would therefore have a much lower ability to maintain stability during the torque peaks. Since thevoltage across the exciter 28 is the voltage across the field windings i8 and I8, sudden changes in the currents in the field windings, which are produced when the angle of displacement changes, produce voltages in the field windings which represent the rate of change of the angle; and the stabilizing or antihunting eflect produced by the transiormer 88 in the control winding 48 prevents hunting of the system. The system may, therefore, be maintained in synchronism over wide range oi! loads and speeds with a minimum normal torque margin, that is, with a minimum excitation inaddition to that necessary to maintain synchronism at any given set oi conditions. The general arrangement of a ship propulsion system arranged to provide stable operationin this manner is described and claimed in' a copending application of Ernst FEW. Alexanderson, Serial 110,477,258, filed before the regulator can take over the control and maintain stable operation. In order to insure starting of the motor without danger of dropping the propeller load, the regulator control windings 4| and 42 are maintained deenergized by the opening of the switch 48 and the switches 81 and 88 are closed to provide minimum resistance of the network 48 so that the winding 48 is energized to a maximum value providing very high excitation for the generator field 18. For induction motor operation, the field I8 is, of course, short circuited through the resistance 28, the switch 21 being opened. This high value 01 excitation is maintained until the motor l2 comes up to the speed at which it may be synchronized. The field excitation is then reduced by opening the switch 81 to cutout the resistance 88 and simultaneously the switch 21 is closed to energize the motor field i8. This provides a relatively high value of excitation during the synchronizing of the motor and generator; and, after a predetermined time delay, which insures synchronizing of the motor and generator, the switch 88 is opened to reduce the excitation of the control field 48 to its minimum or normal value and slmultaneously the switch 48 is closed to place the pilot exciter 84 under control of the regulator In order to effect the starting and reversing operations automatically, there is provided a controller 1| arranged to energize switch operating coils 12, 13, I4, 15, 16 and 11 for operating the switches 24, 25, 21, 28, 45, 51, 88 and I4, respectively. An interlocking coil 18 is provided so that the reversing switch l4 may not be moved from one position to the other unless the coil 18 is energized by closing of the back contacts of the switch 21 in circuit therewith to retract an armature 18. The controller 1| is provided with contacts 88, 8|, 82, 83, 84 and 88 which when closed by cams'88, 81, 88, 89, 88 and 82 energize the coils 12, 13, 14, 15, 18 and 11 respectively. When the controller 1| is moved for starting the ship ahead, it initially places its cams 88, 88 and 88 in position to close its contacts 88, 88 and 84, thereby placing maximum separate excitation on the controlling field winding 48 of the exciter 84 and energizing the generator field winding II to a high value of excitation. Further movement of the controller 1| opens its contacts 84 and closes its contacts 8| by means of cams 88 and 81. This energizes the motor field winding for synchronizing the motor and generator and simultaneously reduces the separate excitation of the control winding 48 to a value which is still substantially above the normal value of excitation. After a predetermined time delay, which may be obtained by regulating the speed at which the controller may be moved to its next point of operation, the contacts 83 are opened and the contacts 82 are closed by the earns 88 and 88 of the controller 1|. This reduces the separate excitation of the control winding 48 to its minimum or normal value and closes the switch 45 to place the exciter it under control of regulator 44. When the controller H is moved to its aster-n position to reverse the motor l2, the controller initially moves its reversing cam 92 to close its contacts 85 and energize the coil 11 to move, the switch H to its reverse position. The remainder of the reverse operation sequence is the same as the forward sequence described above and is controlled by corresponding cams designated by the same numerals as the forward position cams with the addition of the letter "12 The control of the speed of the ship after the con troller H has been operated to synchronize the motor I2 is accomplished by varying the speed of the turbine 9 by operation of the throttle valve 30. It has been found that the arrangement described above for starting and synchronizing the motor and for placing the system under control of the regulator 44 makes it possible to start or reverse the motor in a much shorter time than was the practice heretofore so that a ship provided with the above propulsion system may be reversed to full speed ahead and brought up to full speed astern in a very short time without danger of having the motor drop its load or fall out of synchronism. Furthermore, the operation of the regulator is such that stable operation is obtained for all loads throughout the range of synchronous speeds. Characteristic curves suitable for the requirements of any particular installation may readily be selected by varying the components of the input circuits of the rectifiers 50 and 52, as by changing the value of the reactor 53 and by the use of the,

series resonant circuit 65, 6G, 61 or the condenser 88.

While I have shown a particular embodiment of my invention in connection with a ship propulsion system, other applications will readily be apparent to those skilled in the art. I do not, therefore, desire that my invention be limited to the particular arrangement shown and described, and I intend in the accompanying claims to cover all modifications within the spirit and scop of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric ship propulsion system comprising a variable speed alternating current generator and a synchronous alternating current propeller driving motor connected thereto, field windings for said generator and said motor, a direct current exciter for energizing said field windings, means including a regulator arranged to control said exciter for maintaining a predetermined electrical characteristic during synchronous operation of said motor, means for energizing said exciter independently of said regulator to provide a predetermined high value of separate excitation on said generator field for starting said motor as an induction motor, means for decreasing said separate excitation to an intermediate value and for energizing the field of said motor to synchronize said motor andsaid generator, and means operable at a predetermined time after the energization of said motor field for lowering said separate excitation to its normal value and for connecting said regulator to control the operation of said exciter.

2. An electric ship propulsion system comprising a variable speed alternating current generator having a field winding and an alternating current motor connected to said generator and having a field winding, means for connecting said motor as an induction motor for starting and as a synchronous motor for its normal speed range, means including a cross-armature-reaction-excited direct current exciter for energizing said field windings, a series field winding on said exciter for substantially neutralizing the normal armature reaction of said exciter, first and second and third control field windings on said exciter, means for energizing said first control winding from a source of direct current to determine the polarity of said exciter and for providing high separate excitation during induction motor operation, and means dependent upon the connection of said motor for synchronous motor operation for decreasing said separate excitation and for connecting said second and third control windings to be responsive to the voltage of said generator for differentially energizing said second and third windings to maintain a substantially constant ratio between the voltage and frequency of said generator.

3. An electric ship propulsion system comprising a variable speed alternating current generator having a field winding and an alternating current motor connected to said generator and having a field winding, means for connecting said motor as an induction motor for starting and as a synchronous motor for its normal speed range, means including a cross-armature-reaction-excited direct current exciter for energizing said field windings, a series field winding on said exciter for substantially neutralizing the normal armature reaction of said exciter, first and second and third control field windings on said exciter, means for energizing said first control winding to provide high separate excitation of said generator field during the induction motor starting operation and for reducing said excitation upon energization of said motor field to synchronize said motor and said generator, and means for connecting said second and third control windings for energizing said second and third control windings diiferentially in response to the voltage of said generator and for further reducing the energization of said first control winding a predetermined interval after energization of said motor field winding whereby said exciter is regulated to maintain a predetermined electrical characteristic of said system.

4. An electric ship propulsion system comprising a variable speed alternating current generator and a synchronous propeller driving motor, field windings for said generator and said motor, means including a cross-armature-reaction-excited direct current exciter for energizing said field windings, a series field winding on said exciter for substantially neutralizing the normal armature reaction of said exciter, means including a control field winding on said exciter and responsive to an electrical condition of said system for regulating said exciter for controlling said system to maintain a predetermined electrical characteristic, means including a separately energized control field winding on said exciter for determining the polarity of said exciter and for providing a high value of field excitation, means for energizing said separately energized control field winding to its full value to start said motor as an induction motor and for lowering the excitation of said field upon energization of said motor field to efifect synchronization of said motor, and time delay means for connecting said regulating means to control the operation of said exciter and regulate the synchronous operation of said motor a predetermined interval after the energization of said motor field.

said field windings, a series field winding on said exciter for substantially neutralizing the normal armature reaction of said exciter, first and second and third control field windings on said exciter, means for separately exciting said first control winding from a source of direct current and for providing three steps of excitation, the first of said steps providing abnormally high excitation, the third step providing normal excitation, and the second step an intermediate value of excitation, means applying said first step for starting said motor as an induction motor, means applying said second step' and energizing said motor field winding for synchronizing said motor, and means applying said third step and connecting said second and third control windings to be responsive to the voltage of said generator for differentially energizing said second and third windings to maintain a predetermined electrical characteristic of said system a predetermined interval of time after energization of said motor field winding for placing said system in condition for operation within the range of synchronous speeds of said motor.

6. A variable speed motor control system comprising a variable speed alternating current generator and an alternating current motor connected thereto, a field winding for said generator, means including a cross-armature-reaction-excited direct current exciter for energizing said field winding, a series field winding on said exciter for substantially neutralizing the normal armature reaction of said exciter, first and second and third control windings on said exciter, means dependent upon an electrical condition of said generator for differentially energizing said second and third control windings to maintain a predetermined electrical characteristic of said system, means for connecting said first control winding to a separate direct current source for energizing said first control winding to determine the polarity of said exciter, and means for introducing in the circuit of said first control winding a voltag proportional to the'rate of change of excitation voltage produced by said exciter to produce a damping force and prevent sustained oscillations of said motor and generator.

7. A variable speed motor control system comprising a variable speed alternating current generator and an alternating current motor connected thereto, a field winding for said generator,

means including a cross-armature-reaction-excited direct current exciter for energizing said field winding, a series field winding on'said exciter for substantially neutralizing the normal armature reaction or said exciter, first and second control field windings on said exciter, means dependent upon the voltage of said generator for differentially energizing said first and second.

control windings to maintain a predetermined voltage-frequency characteristic of said system, said means including a first rectifier and an input circuit for said rectifier having a linear impedance-current characteristic for supplying said first control .winding and a second rectifier having a non-linear impedance-current characteristic for supplying said second winding, and means dependent upon variations in the frequency of said generator voltage for varying the impedancecurrent characteristic of one of said circuits in the lower portion of the frequency range 01 said system.

8. A variable speed motor control system comprising a variable speed alternating current generator and an alternating current motor connected thereto, a field winding for said generator, means including a cross-armature-reaction-excited direct current exclter for energizing said field winding, a series field winding on said exciter for substantially neutralizing the normal armature reaction of said exciter, first and second control field windings on said exciter, means dependent upon the voltage of said generator for difierentially energizing said first and second control windings to maintain a predetermined voltage frequency characteristic of said system, said means including a first rectifier and an input circuit for said rectifier Having a linear impedance current characteristic for supplying said first control winding and a second rectifier having a non-linear impedance current characteristic for supplying said second winding, and means including a tuned series resonant circuit connected in shuntv with said second rectifier input circuit for lowering the voltage-frequency characteristic of said system in the lower portion of the frequency range or said system.

9. A variable speed motor control system comprising a variable speed alternating current generator and an alternating current motor connected thereto, afield winding for said generator, means including a cross-armature-reaction-excited direct current exciter for energizing said field winding, a series field winding on said exciter for substantially neutralizing the normal armature reaction of said exciter, first and second control field windings on said exclter, means dependent upon the voltage of said generator for differentially energizing said first and second control windings to maintain a predetermined voltage frequency characteristic of said system, said means including a first rectifier and an input circuit for said rectifier having a linear impedance current characteristic for supplying said first control winding and a second rectifier having a non-linear impedance current characteristic for supplying said second winding, and means including a condenser and a resistance in par-.

allel and connected in series in said first rectifier input circuit for lowering the voltage-frequency characteristic of said system in the lower portion 01' the frequency range of said system.

FREDERICK E. CREVER. 

